DSN is the acronym for the Defense Switched Network. According to the Department of Defense's Defense Information Systems Agency, "The DSN system is used only for official business or in the interest of the government and is the first choice for all switched voice and dial-up video telecommunications between DoD user locations." If you possess the clearance to use the DSN, dialing your call is very straightforward.
Determine the DSN area code you are calling. There are six primary DSN area codes, and each denotes a specific geographical location around the world. As of the date of this publication, the six DSN area codes are: Continental U.S./Puerto Rico - 312 Europe - 314 Pacific - 315 Alaska - 317 Central Command (Southwest Asia) - 318 Canada - 319
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Obtain your access number if required. Your DOD/military command should let you know if an access number is required to make DSN phone calls. Ask your supervisor if you are unsure, but most commands don't require an access number to dial DSN numbers.
Pick up phone and listen for a dial tone. Enter in the access number and listen for dial tone again. Dial the area code and DSN phone number.